The present invention relates to iron(III) orthophosphate, the production thereof and use thereof for the production of LiFePO4 cathode material for Li ion accumulators.
Rechargeable Li ion accumulators are widespread energy accumulators, particularly within the mobile electronics domain, since the Li ion accumulator excels through its high energy density and can deliver a high nominal voltage of 3.7 volts, so that the Li ion accumulator is significantly smaller and lighter than conventional accumulators, but with comparable capacity. Spinels such as LiCoO2, LiNiO2, LiNi1-xCOxO2 and LiMnnO4 have established themselves as cathode materials. In order to increase the safety of Li ion accumulators, above all in respect of thermal overload during operation, LiFePO4 was developed as cathode material. This material excels through improved performance, higher specific capacitance and high thermal stability during operation.
High requirements in respect of purity are placed upon the cathode material of an accumulator, since any contamination which can give rise to undesirable redox reactions during operation (charging or discharging) has an adverse effect upon the performance of the accumulator. The type and concentration of possible contamination is essentially dependent upon the quality of the raw materials which are used to produce the cathode material. In the production process of the cathode material, steps can be taken to subsequently reduce impurities, but generally this is associated with an increase in production costs. It is therefore desirable to use starting materials or raw materials which are as pure as possible for producing the cathode material.
One starting material for producing LiFePO4 for lithium ion accumulators is iron orthophosphate, the purity and structure or morphology of which have a significant influence upon the quality of the resulting cathode material.
Known processes for producing iron(III) orthophosphate use FeSO4 and FeCl3 as starting materials or raw materials, but also organometallic precursor compounds like FeC2O2 (Gmelins Handbuch der anorganischen Chemie, Eisen Teil B, pages 773 ff.; U.S. Pat. No. 3,407,034; C. Delacourt et al., Chem. Mater. 2003, 15, 5051-5058; Zhicong Shi et al., Electrochemical and Solid State Letters 2005, 8, A396-A399). The phosphorus or phosphate components in these starting materials are incorporated by a phosphate salt or phosphoric acid. In the processes described, additions of HCl, NaOH, NH3, NaClO3 or surfactants are always needed in order to control the chemical and physical properties of the products obtained. This means that the materials produced in this way always contain impurities of anions like chloride or sulphate, cations like sodium or ammonium, or organic components. On a large scale, these impurities can only, if at all, be removed by extremely expensive and cost-intensive purifying processes.
Other cationic contaminants, e.g. transition metals which were originally contained in the raw materials used, like FeSO4 or FeCl3, cannot generally be separated or washed out easily because as a rule they form phosphate salts which are not readily soluble, and usually crystallise with the desired iron phosphate.
WO 02/30815 describes a process for the production of LiFePO4 from iron phosphate and lithium, wherein to produce the iron phosphate an iron oxide is dissolved in 85% phosphoric acid with heating. The solution is then diluted until the solubility limit of FePO4 is reached and the material crystallises. By fractionated dilution, undesirable metal phosphates are intended to be separated off which have a smaller solubility product than FePO4. This process has the drawback that it requires a very high energy input, and needs a lot of water for the product to precipitate. With this process, a soluble complex of iron forms which is stable for weeks and crystallises only slowly. This considerably reduces the economic yield of the product. By boiling the solution for several days it is possible to increase the yield, but a very large amount of energy is required. In addition, with this process, a large quantity of diluted phosphoric acid occurs which can only be re-used in the process after it has been concentrated. Therefore, the process is not worthwhile either from an economical or ecological viewpoint.
The prior art processes for producing iron phosphates have further drawbacks if the iron phosphate product obtained is intended for use in the production of LiFePO4 for Li ion accumulators. Important factors with respect to the suitability of the material are the morphology and particle size distribution of the iron phosphates. The prior art processes of precipitating iron phosphate usually give rise to spherical crystals of varying size. However, they have a small surface area in comparison to other crystal morphologies. For use as cathode material in Li ion accumulators an iron phosphate with a large crystal surface is advantageous in order to guarantee that the lithium ions penetrate at high speed and in large numbers. Furthermore, it is advantageous to obtain crystals which are small in size in order to reduce the diffusion paths and diffusion times of the lithium ions. Also, a high bulk density and compressibility of the material is desirable so that a high energy storage density is given in the cathode material produced.
The aim of the present invention was therefore to create an iron phosphate and a process for the production thereof, wherein the drawbacks known from the prior art are overcome, and which has the afore-mentioned desirable properties.